This is likely due to visitors to your site visited your site via two different methods in the same session. For example if a visitor originally reached your site from “keyword A”, navigated back to the search engine, performed another search and clicked on “keyword B” and came to your site again, you would see one visit for “keyword A” in your report and 0 visits for “keyword B”.

http://adwords.blogspot.com/2009/09/announcing-view-through-conversion.html this is an old one but after using it I need to give Google some big props. This can dramatically change the way you value the content network. If your content ad shows to a user and they eventually convert on your site, you’ll know! The old marketing adage “a customer must see your ad 12 times before they remember you” now has a way to measure! I’m excited. More on it here